The labor-backed New Yorkers for Great Public Schools is calling on candidates in next year's city elections to reject donations StudentsFirstNY, the education-reform group backed by former Washington schools chancellor Michelle Rhee, former New York schools chancellor and current New Corp. executive Joel Klein, and Mayor Michael Bloomberg.

Today, Comptroller John Liu became the first (and so far, only) 2013 mayoral contender to promise to do the same: A Liu campaign spokesperson told me they will not accept donations or other support from StudentsFirstNY.

For Liu, who has set himself up as the most stridently anti-Bloomberg Democrat in the field, it's not a particularly risky move.